Metal tubes are roughly classified according to manufacturing method as welded tubes or seamless tubes. A welded tube is manufactured using a metal sheet as a material, by forming this metal sheet into an arc-like shape by press or roll, and joining both ends of the arc by welding into a tubular shape. A seamless tube is manufactured by rendering a solid metal blank or a billet hollow by rolling or extrusion so that it has a desired size. Either is selected according to facility type. However, when pressure is applied to the inside surface of a tube, and damage by tube breaking has a critical influence on the facility, a seamless tube is generally used.
Examples of such a facility include steam pipework and heat exchanger tubing in generating power plants and atomic power plants. In recent years, the temperature has been increased for the purpose of improving power generation efficiency, small-diameter metal tubes for heat exchange have been required to have high quality, and steam pipework needs to deal with high flow (to have a large diameter).
In methods of manufacturing seamless tubes, the upper limit of the diameter of the tube manufactured has been about 400 mm. For this reason, when a metal tube having a larger diameter is needed, a once-manufactured tube is expanded again by a method such as rolling.
Tube expanding methods include a method such that tube expansion is performed using a tool placed between a plurality of mill rolls (referred to as method A for the sake of convenience). Although that method is suitable to mass manufacture products having a specific size, it has the following disadvantages. For example, the facility cost is high, and scratches are prone to be generated on the outer surface of the tube due to contact with the rolls. Thus, that method is not widely used.
Tube expanding methods also include a method such that a tool is placed on the internal diameter side, and a tube is extruded (for example, Japanese Unexamined Patent Application Publication No. 61-56746) or drawn to the large diameter side of the tool (referred to as method B for the sake of convenience). That tube expanding method can be performed in both heated and non-heated manners. When the thickness of a tube after tube expansion is to be reduced to about 5% of the external diameter, it is performed in a heated manner from the viewpoint of production efficiency. That method has the advantage that power can be saved from the viewpoint of processing, and the degree of freedom of tube size is relatively high. On the other hand, a certain level of skill or experience is required to manufacture with this method. The reason is as follows. In the above-mentioned method A, the external diameter side can be fixed with the roll, and the internal diameter side can be fixed with the tool, and thus the size of finished product can be easily achieved. In the method B, since there is no tool on the external diameter side, the flexibility of deformation is high, and not only the external diameter but also the thickness and the internal diameter cannot be determined. Thus, determining the forming conditions such as tool shape is generally difficult.
To eliminate defects of products caused by such high flexibility of deformation, a tool shape that can reduce the bentness of products has been proposed (for example, Japanese Unexamined Patent Application Publication No. 2001-113329).
However, in Japanese Unexamined Patent Application Publication No. 2001-113329, there is no technical disclosure of the basic point of how to manufacture the product having a predetermined product dimension. In other words, on the premise that conditions of forming for a predetermined size product are known, Japanese Unexamined Patent Application Publication No. 2001-113329 only states that it is effective to change the tool shape.
Thus, in the method B (method such that the internal diameter side of a tube is passed through a tool by pressing or drawing the tube end), determining the manufacturing conditions such as an appropriate tool shape and/or hollow piece size according to arbitrary product dimension requires many trial productions and tests, and is extremely difficult and not economic. This is a great disadvantage. For this reason, it is difficult to reflect customer needs, and method B is not widely used.
It could therefore be helpful to provide a tube expanding method for manufacturing a metal tube, including passing the internal diameter side through a tool by pressing or drawing the tube end of a metal tube, wherein to obtain a finished product having an arbitrary size, manufacturing condition of tube expanding method (tool shape, hollow piece size) such that the analysis result falls within a specific limited range is obtained using simulation, and thereby an increase in product dimension, a reduction in testing time and cost reduction which have been problems in manufacturing, and industrialization can be easily achieved.
Although the manufacturing method and chemical composition of a metal tube used in our tube expanding method are not particularly limited, the metal tube shown below is particularly preferable.
(1) JISG 3458 alloy steel pipe for piping (STPA20, STPA22 to 26) (corresponding to ASTM A335, ASTM A405, BS 3604, DIN 17175, DIN 17177):
The preferred composition range is, C: 0.10-0.15%, Si: 0.10-1.00%, Mn: 0.30-0.80%, P: 0.035% or less, S: 0.035% or less, Mo: 0.40-1.10%, Cr: 0% or 0.50-10.00%. As necessary, Cu: 1% or less, Ni: 2% or less, Nb: 0.1% or less, V: 0.5% or less, Ti: 0.2% or less, B: 0.005% or less, REM: 0.02% or less, Ca: 0.01% or less can be added. As unavoidable impurities, N: 0.010% or less, O: 0.006% or less are permissible. In terms of manufacturing method, a seamless steel tube manufactured by Mannesmann type piercing is preferable.
(2) JISG 3462 alloy steel pipe for boiler and heat-exchange (STBA12 to 13, STBA20, STBA22 to 26) (corresponding to ISO 9329-2:1997, ISO 9330-2:1997, ASTM A161, A199, A179 A200, A209, A213, A250, A423, BS 3059, DIN 17175, DIN 17177):
Preferable composition range is, C: 0.10-0.20%, Si: 0.10-1.00%, Mn: 0.30-0.80%, P: 0.035% or less, S: 0.035% or less, Mo: 0.40-1.10%, Cr: 0% or 0.50-10.00%. As necessary, Cu: 1% or less, Ni: 2% or less, Nb: 0.1% or less, V: 0.5% or less, Ti: 0.2% or less, B: 0.005% or less, REM: 0.02% or less, Ca: 0.01% or less can be added. As unavoidable impurities, N: 0.010% or less, and O: 0.006% or less are permissible. In terms of manufacturing method, a seamless steel tube manufactured by Mannesmann type piercing, or an electric resistance welded steel pipe manufactured by electric resistance welding using high-frequency current is preferable.